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Heat periods are a challenge for everyone

Pigs sweat through their snouts and dogs through their paws. To cool down, they have to find a shady spot—ideally in mud or on a cool, shaded tile. Cows can sweat—we notice this from their damp coats. But this is far from enough. When temperature and humidity are high, they stand in the barn with open mouths, panting like dogs. At the same time, feed intake, rumination, and milk production decrease. These are the visible signs of heat stress—and they should give us pause. Because much more is happening inside the cow’s body.

Sensitive days

In 2019, a study was published analyzing data from more than 22,000 calving cows with regard to retained placenta and other periparturient problems. Especially when heat stress occurred within the first five days after calving, health disorders increased linearly with the THI (temperature-humidity index). Further studies on the occurrence and severity of uterine inflammations also showed a general linear relationship with THI.

Case 1: Immune system

Many farms have already experienced this: high temperatures are followed by an increase in somatic cell counts. This is not only due to increased activity of environmental pathogens, but also to the negative effects on immune defense. The “law enforcers” (macrophages) of the immune system normally recognize foreign substances and call in the “special unit” (phagocytes, leukocytes). These specialized cells fight the pathogens and neutralize them. Under heat stress, however, the whole team becomes sluggish. Fewer phagocytes are activated, allowing pathogens to invade tissues and organs more easily.

Crime scene: udder

The udder is often the first victim of these attackers. Milk is an excellent nutrient medium, and the teat canal is an open gateway for all kinds of pathogens at least twice a day. In addition, the udder comes into contact with a particularly large number of germs from different sources (housing and milking equipment).

Case 2: Hormones

Hormones (messenger substances) regulate many bodily functions. Interestingly, the first of them—adrenaline—was only discovered in 1901. They are released into the bloodstream from their site of origin (e.g., thyroid, ovaries) and trigger specific reactions once they bind to their target. Under heat stress, the secretion of all reproductive hormones decreases, and processes become disrupted. These include, among others, estrogen, progesterone, and prostaglandin.

Crime scene: uterus

Without proper signaling from these “messengers,” fewer follicles mature, fertilized eggs may not even reach the uterus, or they fail to implant in the uterine wall. Even if the correct time for insemination is identified—despite cows being less active in heat—higher embryonic losses must be expected. If heat stress occurs during early pregnancy, it affects not only the cow but also the developing fetus. As temperatures rise, blood flow is redirected toward the skin surface to dissipate heat, which reduces nutrient supply to the fetus. Even though calves are born with virtually no immune system, the organs necessary for developing immunity form within the first 60 days after conception.

Case 3: Interconnections

To make matters worse, some hormones also influence the body’s immune defense. Estrogen, for example, interacts with immune cells by activating their function and encouraging them to perform effectively. Lower estrogen levels due to heat stress therefore mean a weaker immune response. The milk-producing hormone prolactin and the stress hormone cortisol—both increasingly released during heat—also have a suppressive effect on the immune system. And that brings us right back to Case 1.

Taking action – but how?

Shade and all structural measures in the barn that contribute to cooling are definitely justified. In various studies, heat-stressed animals were compared with “cooled” groups, and clear effects were usually visible—right down to the blood and tissue level. Measures to reduce stress can also be applied through nutrition: due to sweating, cows lose not only water but also minerals. These losses should of course be compensated. Therefore: increase mineral supplementation and keep water sources clean. Antioxidants help protect animal health, while live yeasts support feed intake and help animals cope better with heat. A combination of several measures—both “external” and “internal”—is certainly the best way to guide the herd safely through the summer.